Polymethylenic sequences

The y relaxation takes place at the lowest temperature, overlaps with the )3 relaxation (Fig. 15), and coincides in location and activation energy with the typical y relaxation of polyethylene [35,36], and also of polyethers [37], and polyesters [38] with three or more consecutive methylene units. It appears, for 3 Hz and tan6 basis, at - 120°C (P7MB) and - 126°C (P8MB), and its location and activation energy (35-45 kJ mol ) agree with the values of a similar relaxation associated with kink motions of polymethylenic sequences. 

It is usually considered that the y relaxation arises from crankshaft and kink movements of polymethylenic sequences, but the clear maximum of tanS and loss modulus for the three polybibenzoates here reported leads to the conclusion that the motion responsible of this relaxation also takes place when one of the methylenic... 

The family of polymers that we refer to as nylons consists of molecules composed of amide groups alternating with short runs of methylene units. These molecules are also known as polyamides, which may be shortened to PA. The generic chemical structure of a nylon molecule is shown in Fig. 23.1. Variations on this basic structure include the length of the polymethylene sequences and the orientation of the amide groups relative to their neighbors. Figure 23.2 shows the chemical structures of nylon 6 and nylon 66, which are the two most common types of nylon. 

Nylon molecules contain short, non-polar, polymethylene sequences and polar amide groups. The ratio of the polar to non-polar components strongly influences the properties of the polymer. We can envisage a vast number of polymers that fall into the class of nylons, but... 

The conformation of the spacer was estimated by using samples carrying a perdeuterated polymethylene sequence. The following expression may be adopted for the deuterium quadrupolar splittings (Av,-) due to the ith methylene unit ... 

Other Aliphatic Polyamides. They are mainly polyamides having longer polymethylene sequences. Their marketing was dictated by the need of technical polymers whose mechanical characteristics are less sensitive to the hygrome-try of the ambient conditions than those of PA-6 and PA-6,6. It concerns PA-6,10, which is obtained by the polycondensation between hexamethylenediamine and sebacic acid [HOOC-(CH2)8-COOH], PA-11 and PA-12. 

In the usual chemical formulas written for chain polymers the sue-cessive units are projected as a co-linear sequence on the surface of the sheet of paper. This form of representation fails to convey what is perhaps the most significant structural characteristic of a long polymer chain, namely, its capacity to assume an enormous array of configurations. This configurational versatility is a consequence of the considerable degree of rotational freedom about single bonds of the chain. In the simple polymethylene chain, for example, the conventional formula... 

The distribution of chain sequence extension, calculated by using RIS models, is compared with isotropic-nematic transition characteristics for a number of thermotropic polymers comprising rigid groups connected by polymethylene spacers. The distribution depends strongly not only on the odd-even character of the number of methylene units of the spacers, but also on the specific groups (or atoms connected at the ends of polymethylene spacers. 

Some of the kinked units discussed in the previous section, such as —0— and — S—, are simple examples of flexible spacers that space the meso-genic units into separate structural sequences of smaller length-to-diameter ratios. Other segments often used as flexible spacers include oligomeric polymethylenes, polyoxyethylenes, polysiloxanes, and so forth. The following series of molecules, (3.5) with n = 1-8, make up one of the oldest examples of the flexible spacer concept in the molecular design of liquid crystals (Vorlander, 1927) ... 

In this article, in connection with our previous endeavor, we would like to report synthesis and properties of new copolyesters having ordered comonomer sequences consisting of mesogenic units and polymethylene spacers, i.e., thermotropic aromatic-aliphatic copolyesters having sequentially ordered structures ... 

This explanation cannot be extended, however, to the sequence XIV R"—R, XV R (CH2)2R, XVI R"(CHj)4R", XVII R (CH2)8R , since these compounds decrease in precipitating power in this order. A structural interpretation of this result in terms of the lack of rigidity of the polymethylene chain might be developed, but it is not very convincing. 

---